Conventional filtration systems for sewage purification consist of filtration modules with a tubular or box-shaped housing in which a plurality of flat filter elements are arranged, spaced apart, parallel to one another. The interspaces between adjacent flat filter elements form throughflow passages. A liquid to be filtered, designated as raw liquid or retentate, is conducted via feed lines or through orifices in the housing walls into the interior of the tubular or box-shaped housing, flows over the flat filter elements and flows out through discharge lines or further orifices in the housing walls. The flat filter elements are configured as pads or cartridges and comprise a flexible drainage layer or rigid drainage plate which is surrounded on both sides by filtration membranes. The flat filter elements have run-off orifices, to which are connected lines through which filtered liquid, designated below as permeate, is discharged.
EP 0 707 884 A1 (whose United States equivalent is U.S. Pat. No. 5,626,752) discloses an arrangement for the filtration and separation of, in particular, biologically organic flow media by reverse osmosis and micro, ultra and nano filtration, with a pressure-tight housing, with an inlet for the flow medium and with outlets for the retentate and the permeate and with a number of filter elements which are arranged in the housing and are spaced apart from one another and which are designed in the manner of a membrane pad and around which the flow medium flows, a number of separate stacks of membrane pads being arranged one behind the other or next to one another in the housing, and the flow medium flowing around the stacks one behind the other or next to one another.
EP 0 129 663 A1 teaches a membrane pad for water desalination by reverse osmosis, ultrafiltration, hyperfiltration, gas permeation and the like, in which a drainage layer is arranged between two outer membranes and the drainage layer is welded continuously, and pressure-tight, to the membranes in a marginal zone.
WO 03/037489 A1 (whose United States equivalent is U.S. Patent Application No. 2005/000881) describes a filtration module for the purification of sewage, with a plurality of filter membrane pockets which have at least one orifice for dewatering their inner space and which are arranged vertically, parallel and preferably at an equal distance from one another in a rigid holder so that a liquid is capable of flowing intensively through the interspaces lying between adjacent filter membrane pockets.
Some of the known filtration modules have a box-shaped housing, with two or four side walls which are arranged opposite one another in pairs and which delimit a spatial volume having a rectangular flow cross section. Flat filter elements are mounted in the housing in such a way that they form a stack which subdivides the spatial volume surrounded by the housing into a plurality of flow ducts of equal size. The purpose of receiving and holding the flat filter elements is served, for example, by a number of grooves, arranged in pairs and parallel to one another, in two side walls of the housing which are arranged opposite one another. Marginal portions of the flat filter elements are fixed in the grooves by means of adhesive joints or mechanical holding devices.
Conventionally, the raw liquid is routed by means of a pump through the filtration module in the vertical direction from the bottom upward. In particular, air-operated mammoth pumps (compressed-air lifts) are used. In this case, compressed air is introduced from a compressor, via compressed-air lines below the filtration modules, into the raw liquid. At the location of introduction, the density of the raw liquid is greatly reduced by the air introduced, and, due to the buoyancy, a flow directed from the bottom upward is formed. The compressed-air lines are equipped, as a rule, with a multiplicity of outlet nozzles. Expediently, the outlet nozzles are arranged uniformly in a horizontal area in such a way that the outflowing air forms a rising bubble carpet which partially or completely fills the flow cross section of the filtration modules.
When a filtration system is operating continuously, particles, the diameter of which is too large to pass through the pores of the filtration membrane, are retained on the surface of the filtration membranes and partially remain in adherence. As a result of the accumulation of such particles over a lengthy period of time, a filter cake builds up, which increasingly blocks the membrane surfaces and reduces the filtration capacity of the system.
In order to restore or reacquire the filtration capacity, the membrane surfaces are regularly cleaned within the framework of plant maintenance. For this purpose, the membrane surface is cleaned mechanically, for example by means of brushes or a water jet or by means of backwashing. The mechanical cleaning is sometimes also accompanied by the use of chemical cleaning agents (chemomechanical cleaning).
During assembly and maintenance, and also during operation, sometimes considerable mechanical forces act upon the filtration arrangement. This applies particularly when a large filtration module with a filtration area of more than 30 m2 is lifted for maintenance purposes out of a liquid to be filtered. The liquid absorbed and retained by the porous filtration membranes increases the weight of the filtration module to considerable extent. Moreover, in a multiplicity of applications, such as, for example, in sewage treatment plants, the liquid to be filtered contains sludge of higher viscosity which adheres to the surface of the flat filter elements and, in practice, does not flow off when the filtration arrangement is lifted as a consequence of maintenance. This adhering sludge, which fills the interspaces virtually completely especially in the case of closely packed flat filter elements, may have a weight of up to a few hundred kilograms. When the filtration arrangement is being lifted as a consequence of maintenance, its load-bearing construction must therefore withstand a load of up to one ton.
In order to ensure the mechanical stability or rigidity necessary for this purpose, the side walls of the filtration arrangement are reinforced and connected to one another by means of a frame or bracings consisting of a stable material, such as, for example, steel. Frame structures of this type are complicated and costly to produce and, moreover, increase the dead weight of the filtration arrangement.